Electrical apparatus



March 13, 1962 A. ROSS 3,025,452

ELECTRICAL APPARATUS Filed Feb. 6, 1958 3 Sheets-Sheet 1 ALBERT ROSS BYE 4 g/ H ATT RNEYS March 13, 1962 A. Ross ELECTRICAL APPARATUS Filed Feb. 6, 1958 3 Sheets-Sheet 2 INVENTOR ALBERT ROSS HIS ATTORNEYS March 13, 1962 A. ROSS 3,025,452

' ELECTRICAL APPARATUS Filed Feb. 6, 1958 5 Sheets-Sheet 3 1%? 3 FIG. l4

a 120 124 I INVENTOR ALBERT R0 LM (,8.

IS ATTORNEYS United States Patent 3,025,452 ELECTRICAL APPARATUS Albert Ross, Dayton, Ohio, assignor to Standard Electrical Products Company, a corporation of Ohio Filed Feb. 6, 1958, Ser. No. 713,690 9 Claims. (Cl. 323-47) This invention relates to an electrical apparatus. The invention relates more particularly to a transformer. The invention relates still more particularly to an adjustable transformer.

In the conventional adjustable transformer having a toroidal winding, a commutator or contact surface upon the winding is used which is created by providing each turn or convolution with a contact portion for engagement by a brush structure which slidingly engages the commutator or contact surface. It is desirable that the brush structure never completely disengages the winding during the movement of the brush structure from one turn of the winding to an adjacent turn thereof. Therefore, it is necessary that during a portion of its movement the brush structure is in engagement with two or more contact portions on the commutator surface. Thus, one or more convolutions of the winding are short circuited by the brush structure during the engagement thereof with the plurality of contact portions.

It has been customary to use one or more high resistance brushes electrically connected together to form a brush structure for contacting the commutator surface of the winding. Such a brush structure has sufficiently high resistance so that when the brush structure is in engagement with a plurality of contact portions of the winding, the current through the shorted convolution or convolutions never exceeds a certain tolerable value. Since however, the load current of the transformer must flow through the brush structure, increasing the resistance of the brush structure increases the losses in the brush structure due to the load current. These losses are, of course, objectionable.

This conventional high resistance brush structure has other objectionable features. One of the objectionable features is that there is considerable heating caused in the brush structure as a result of the load current and short circuit current therein.

Most of the heat from the brush structure is transferred to the winding, causing increased heating in the winding. This problem. becomes much more pronounced as power ratings increase.

It has been customary in a toroidal type of winding to use wire which has a round cross-section. The turns or convolutions of round wire lie side by side at the commutator surface. However, a toroidal winding having a reasonable number of turns of round wire occupies a rather large space and thus results in a transformer of rather large physical dimensions.

The commutator surface of a winding is usually provided by a grinding or machining operation. As such machining occurs for forming a commutator surface on round wires, the width of the commutator portion of each convolution increases. This often results in commutator portions of uneven width.

It is an object of this invention to provide an adjustable transformer in which the brush structure does not cause increased heating in the winding of the transformer.

It is another object of this invention to provide an adjustable transformer having a winding capable of transforming large amounts of power in consideration of the physical size of the transformer.

Another object of this invention is to provide an adjustable transformer which is capable of operating under high temperature conditions.

Another object of this invention is to provide an adice justable transformer in which the current carrying capacity of the winding may be increased without increasing the size of the core of the transformer.

Another object of this invention is to provide a method of construction of a winding for an adjustable high current and/or high power transformer.

It is another object of this invention to provide a toroidal winding in which the width of the commutator portion of each convolution does not change during the machining operation which forms the commutator surface.

Other objects and advantages reside in the construction of parts, the combination thereof, the method of manufacture, and the mode of operation, as will become more apparent from the following description.

In the drawings,

FIGURE 1 is an over-all perspective view, with parts broken away, showing an adjustable autotransformer of this invention.

FIGURE 2 is a top view, with parts shown in section and with parts broken away, of an adjustable auto-transformer of this invention.

FIGURE 3 is a sectional view taken substantially on line 3-3 of FIGURE 2.

FIGURE 4 is an enlarged sectional view taken substantially on line 44 of FIGURE 2.

FIGURE 5 is an enlarged fragmentary top view showing a portion of the winding and brush structure of an adjustable auto-transformer of this invention.

FIGURE 6 is a view taken substantially on line 6-6 of FIGURE 5.

FIGURE 7 is a side elevational view, with parts broken away, of a portion of a winding of a transformer of this invention.

FIGURE 8 is a sectional view taken substantially on line 8-8 of FIGURE 7.

FIGURE 9 is an elevational view showing a portion of a winding of a transformer of this invention.

FIGURE 10 is a front elevational view showing a modification of a portion of a transformer of this invention.

FIGURE 11 is a fragmentary top view showing a modification of another portion of a transformer of this invention.

FIGURE 12 is a fragmentary top plan view showing another modification of a transformer of this invention.

FIGURE 13 is a sectional view taken substantially on line 13-13 of FIGURE 12.

FIGURE 14 is a sectional view taken substantially on line 14-14 of FIGURE 13.

FIGURE 15 is a view taken substantially on line 15-15 of FIGURE 12.

Referring to the drawings in detail, an adjustable autotransformer of this invention comprises a base 16 to which is attached an annular core 18 in a manner discussed below. Disposed upon the annular core 18 is a winding 20 which is toroidal in form, having a plurality of turns or convolutions.

Each convolution of the winding 20 is spaced from its adjacent convolutions and comprises conductor pieces joined end to end and firmly attached one to the other. As best shown in FIGURES 7 and 8, each convolution of the winding 20 may comprise an offset U-shaped portion or piece 24 and a link 26.

The portions or pieces 24 and the links 26 are stamped, cast, or otherwise formed from sheets of conductor material. However, the winding 20 may comprise joined pieces or portions of other shapes or configurations.

Each U-shaped portion 24 has a leg 23 and a leg 25. As shown in FIGURES l, 3, 7, and 8, a part of each leg 23 is slightly bent or offset. The link 26 is shown as having a groove at each end thereof which is adapted to firmly receive tapered portions 28 extending laterally from 3 the lower portion of the legs 23 and of adjacent U-shaped portions 24, as shown in FIGURES 7 and 8. Each of the links 26 joins the leg 23 of a U-shaped portion 24 to a leg 25 of an adjacent U-shaped portion 24.

A pair of rigid flat rings of non-conductor material are disposed at opposite portions of the core 18 in engagement therewith and serve to insulate the winding 20 from the core 18, as shown in FIGURES 1 and 3. Each of the rings 30 extends slightly from the core 18 and is notched to receive the convolutions of the winding 29. The notches of the rings 30 maintain the desired spacing between adjacent convolutions of the winding 20.

The winding 20 is applied to the core 18 by placing the U'shaped pieces 24 upon the core 18 as each U-shaped piece 24 is partially disposed within a groove of each ring 30. Each U-shaped portion 24 thus forms a part of a turn or convolution of the winding 20.

One of the links 26 is then used to attach one U-shaped portion 24 to an adjacent U-shaped portion 24. The tapered portion 28 at each end of each link 26 fits into a groove of a leg 23 or 25 of the U-shaped portions 24, as discussed above. These portions 24 and the links 26 may also be secured one to the other by any suitable means, such as solder or the like.

After all of the portions 24 and the links 26 are attached in end to end relation forming convolutions around the core 18, the core 1% and the winding 24) are applied as a unit to the base 16.

An insulator sheet 34 separates the winding 26 from the base 16 as an edge of each convolution of the winding 20 rests upon the insulator sheet 34, as shown in FIG- URES l and 3. A clamp plate 36 of insulator material engages the upper edge portions of the winding 20, as shown in FIGURES 1, 2, and 3. Bolt members 38 extend through the clamp plate 36 and into the base 16 for firmly retaining the core 18, the winding 26, and the insulator sheet 34, with relation to the base 16.

As shown in FIGURES 1 and 2, the winding 20 has an end 40 and an end 42 extending somewhat radially from the core 18 to a terminal board 44. A connector or terminal bolt 46 is attached to the end 49 and a connector or terminal bolt 48 is attached to the end 42 of the winding 20. A

A contact plate or conductor plate 51 is firmly attached to the clamp plate 36 by means of bolts 52. The conductor plate 51 has a radial stem 53, in the same plane therewith, which is joined to the terminal board 44 by means of a connector or terminal bolt 55.

Rotatably extending through the clamp plate 36 and the conductor plate 51 is a shaft 56 which is journalled in the base 16. A wheel 57, shown in FIGURES 1 and 3, is used for rotation of the shaft 56. Rigidly attached to the shaft 56 is a hub 58 and a collar 59, integral therewith, both of insulator material. A set screw 61 may be used to aid in securing the collar 59 to the shaft 56. The hub 58 is provided with grooves 60 at opposite portions thereof and normal to the shaft 56, as shown in FIGURES 1 and 4. Pivotally carried within the grooves 60 by means of pins 62 are a pair of arm members 64 which extend from opposite portions of the hub 58. As shown in FIG- URE 4, the width of the arm members 64 is somewhat less than the width of the grooves 60 so that the arm members 64 may pivot through a limited angle within the grooves 60.

A pair of springs 68 are carried by the hub 58 and engage the arms 64 for exerting a pivotal force upon the arms 64 as shown in FIGURE 4. Each of the springs 68 is compressed below a cap screw 69.

The arms 64, extending in one direction from the hub 58, converge and extend to the outer periphery of the winding 20, as shown in FIGURES l and 2. The converging portions of the arms 64 are firmly mechanically attached one to the other by any suitable non-conductor means. Herein, an insulator collar 67, as shown in FIG- URES 1, 2, and 3, encompasses the arms 64. The arms 64 are shown as being conductor members and have attached at the end thereof brush holders 70 which are in firm contact with a pair of low resistance brush members 72, as best shown in FIGURE 6. The brushes 72 slid ably engage contact portions or commutator portions of the winding 20. Each convolution of the winding 20 has a commutator or contact portion engageable by the brushes 72. The brushes 72 may be soldered or otherwise attached to the holders 70, as shown by reference numeral 71, to insure a good electrical contact. Suitable screw members 72 may be used to firmly secure the brush holders 70 to the arms 64. An insulator strip 76 separates the brushes 72 one from the other and also separates the arms 64 one from the othe at the end thereof, as shown in FIGURE 6.

The brushes 72, as shown in FIGURE 6, are of such dimension with respect to the convolutions of the winding 20 that neither brush 72 may contact more than one commutator portion of the winding 20 at any given time. However, the brushes 72 are so arranged one with respect to the other that both of the brushes 72 may be in contact with a single commutator portion of the winding 20. Furthermore, one of the brushes 72 may be in engagement with a contact portion of the winding 20 while the other brush 72 is disposed intermediate two adjacent contact portions of the Winding 20. One brush 72 may contact one commutator portion while the other brush 72 engages an adjacent commutator portion, as shown in FiG- URE 6.

Due to the fact the relative positions of the brushes 7-2 and the arms 64 are firmly maintained by the collar 67, and since at least one of the brushes 72 is always in engagement with a contact portion, the other brush is positioned in the plane of the commutator portions even when disposed intermediate adjacent convolutions.

From the foregoing discussion it is understood that with rotation of the shaft 56 the hub 58 also rotates. With the hub 58 the conductor arms 64 move, carrying therewith the brush members 72 as the brush members 72 contact the winding 20. The brush members 72 are urged into engagement with the winding 20 by means of the spring 68 which is shown in FIGURE 4 and which urges movement of the arm 64 toward the winding 26.

The conductor arms 64, extending from the hub 58 in the opposite direction from the brush holder 70, are electrically connected to a resistor or electrical impedance 78 which is disposed between the arms 64, as shown in FIGURES l and 2. The impedance or resistor 78 is carried by insulator support rods 81. A central portion of the resistor 78 is electrically attached by means of a bolt 83 to a connector bar which extends from the impedance 78 to the hub 58.

At least one of the brush members 72 is always in contact with a commutator portion of the winding 29. As the brush members 72 move along commutator portions of the winding 20, the brushes 72 provide movable taps upon the winding 20. When the brushes '72 are disposed in engagement with adjacent commutator portions of the winding 20, as shown in FIGURE 6, the intermediate convolution is shorted through the impedance 78. This is due to the fact that each end of the resistor 78 is electrically connected to one of the brushes '72 through its respective arm 64. Therefore, the short circuit current flows through the impedance 73. Due to the fact that the resistor 78 is disposed remotely from the winding 26, current flow through the impedance '78 does not cause additional heating to the winding 26. Furthermore, the impedance 78 may be disposed in such a manner that the impedance 78 is easily and readily cooled by convection or otherwise.

As shown in FIGURE 3, movably carried by the bar 89 and electrically connected thereto is a contact pin 84 which is resiliently urged into engagement with the contact plate 51 by means of a spring The spring 86 is partially enclosed by a cover 87. The contact pin 84 carries current between the bar 80 and the contact plate 51.

Due to the fact that the connector bar 80 joins the resistor 78 at the central portion thereof, current from either brush 72 to the connector bar 80 flows through only one-half of the impedance 78. The value of the impedance or resistor 78 is selected to be large enough to limit the short circuit current between adjacent contact portions to a tolerable value and to be small enough to avoid high losses and voltage drops due to the flow of load current through the divided portions of the resistor. The value of the impedance or resistor 78 is thus preferably so selected that a minimum over-all power loss is caused under load conditions.

Due to the fact that the contact plate 51 has the stem 53 thereof connecting to the terminal bolt 55, the brush members 72 are thus electrically connected through the arms 64, and the impedance 78, to the terminal bolt 55.

Any suitable externally extending conductor members (not shown) are thus attached to the terminal bolts 46, 48, and 55 for transmission of electrical energy to and from the auto-transformer.

FIGURE 9 shows a portion of a helical or toroidal winding of this invention in which pieces of material are attached one to the other in the manner shown in FIGURES 7 and 8.

- FIGURE shows a portion of an auto-transformer of this invention in which a winding 90 upon a core 91 is engaged by a pair of brush members 92. The brush members 92 are separated by means of an insulator spacer 94 and are firmly attached to holding means 96. The holding means 96 may be of insulator material, used for carrying the brush members 92 along the winding 90.

Each of the brushes 92 is provided with a pigtail 100 which is connected to a suitable current limiting resistor (not shown). Thus, in the structure shown in FIGURE 10 the support arms 96 serve only as support members and do not also serve as conductor members for carr.y ing current to and from the brushes 92.

FIGURE 11 shows a winding 102 of this invention which comprises rigid conductor pieces which may be attached together in the manner shown in FIGURES 7 and 8 to form convolutions. The winding 102 may also comprise rigid pieces of any other suitable dimensions joined in end to end relation. The winding 102 is disposed upon an annular core 103. The portion of each convolution which extends substantially radially with respect to the core 103 is tapered. Thus, as shown in FIGURE 11, compared to FIGURE 5, for a core of a given size a larger number of convolutions are provided upon the core when the portions of each convolution extending toward the center of the core are tapered.

The tapered pieces or portions of the winding 102 are made by casting or stamping or in any other suitable manner, as discussed with respect to the pieces shown in FIGURES l, 2, 5, 7, 8, and 9. As is well known in the art of auto-transformers, it is ofen very desirable to have a large number of turns or convolutions of a winding within a given dimension. Voltage between turns or convolutions is decreased if there are a larger number of turns With the same voltage applied to the winding. This decrease in voltage between turns may thus permit a decrease in cross-section of the core and thereby permit a decrease in the length of each convolution. Conversely, by maintaining the total number of turns and by tapering the turns, as shown in FIGURE 11, the length of the core may be reduced.

In FIGURE 11 a pair of brushes 106 are shown in sliding engagement with the winding 102.

In FIGURES 12, 13, 14, and an adjustable autotransformer of this invention is shown provided with means for automatically causing the brush members thereof to be normally positioned in a given manner with respect to convolutions of the transformer winding. A pair of brush members 110 and 112 are retained by brush holders 114 so that the brush members and 112 are firmly electrically connected to conductor arm members 113 and -115 respectively. The arm members 113 and 115 are mechanically connected to a hub 116 which rotates with a support shaft 117. The conductor arm members' 113 and 115 and the brushes 110 and 112 are elec trically separated one from the other by means of an insulator strip 118, shown in FIGURES 12 and 15, but the relative position of the arm members 113 and 115, one with respect to the other, is mechanically maintained by any suitable means. A winding 120 is positioned upon a core 122 provided with an insulator 124. The winding 120 has spaced-apart convolutions which are engaged by the brush members 110 and 112. A resistor 130, carried by insulator rods 131, is electrically connected between the conductor arms 1.13 and 115, as shown in FIG- URE 12.

Attached to the arms 1'13 and 115 intermediate the brush holder 114 and the resistor 130, is a support plate 134, as shown in FIGURES 12 and 13, which preferably consists 'of non-conductor material. The support plate 134 is attached to the arms 113 and 115 by means of bolts 135 and is disposed adjacent a portion of the winding 120.

Attached to the support plate 134, intermediate the arms 113 and 115 is a carrier or holder 138. A vertical shaft is movably retained within the carrier 138 and has a yoke 142 integral therewith at the lower end thereof, as shown in FIGURE 14. The yoke 142 engages a shaft 144 of a roller 146. The shaft 144 extends through vertical slots 148 of the holder 138, as shown in FIG- URES 13 and 14. A spring 150 engages the yoke 142 and urges the roller 146 into engagement with the winding 120.

A portion 151 of each convolution of the winding 120 is tapered or coined and extends slightly from the main portion of the convolution for providing a narrow edge for engagement by the roller 146, as shown in FIGURES l3 and 14. The roller 146 therefore cannot normally position itself in engagement with only one convolution of the winding 120. Thus, the roller 146 normally rests in engagement with two adjacent convolutions of the winding 12%, as shown in FIGURE 13.

The holder 138 is so attached to the arms 113 and 115 that when the roller 146 is disposed in engagement with two adjacent convolutions of the winding 120-, as shown in FIGURE 13, the brush members 110 and 112 are disposed so that only the brush 112 is in engagement with a convolution of the winding 120, as shown in 'FIG- URE 15.

Thus, as the brush members 110 and 112 are moved by means of the conductor arms 113 and 115 along the convolutions of the winding 120, each of the brushes 110 and 112 is momentarily in and out of engagement with the winding 120. However, one of the brushes 110 or 112 is always in engagement with the winding L20. The resistor 130 limits the current flow between adjacent convolutions when the brushes 110 and 112 are in engagement with adjacent convolutions. When the arms 1 13 and 115 are at rest, the roller 146 automatically assumes its position, as shown in FIGURES 13 and 14. Thus, when the arms 113 and 115 are at rest, the brushes 110 and 112 are automatically positioned so that the brush 112 is in engagement with a convolution of the winding 120 and the brush 110 is out of engagement with the winding 120, as shown in FIGURE 15. Therefore, the resistor 130 carries no current when the brushes 110 and 112 are at rest. Thus, only the arm member 115 which is connected to the brush 112 carries current when the brushes 112 and 110 are .at rest. Therefore, an output lead such as the lead is connected only to the arm 115, as shown in FIGURE 12. When the brushes 110 and 112 are at rest there is no current flow through the resistor 130. Therefore, there is no power loss or heating in the resistor 130 when the brush members 110 and 112 are at rest. The output lead 160 has an end carried by the hub 116 and in engagement with a contact plate or conductor plate 162, which may be similar to the conductor plate 51 shown in FIGURES l, 2, and 3.

Due to the fact that the convolutions of the Winding of a transformer of this invention may be constructed of suitably shaped portions joined together, the dimensions of each convolution of the winding may be so selected that large conductors with excellent cooling may be constructed without increasing the size of the core. For example, as shown in FIGURE '7, the leg is consider ably wider than the leg 23. The leg 25 is outside the core and therefore increase in the width of the leg =25 does not disturb other dimensions of the winding or core. Increase in the Width of the leg 25 decreases the over-all resistance of the Winding and therefore decreases the copper losses while such increase in width increases the available cooling surface of the winding. Furthermore, by providing a winding having tapered portions, as shown in FIGURE 11, the width of an inner leg portion, such as the leg 23 shown in FIGURE 7, may be further increased without increasing the size of the core.

Thus, it is readily understood that the transformer of this invention may have a much larger rating than a conventional transformer in a given physical size.

Although the preferred embodiment of the device has been described, it will be understood that within the purview of this invention various changes may be made in the form, details, proportion and arrangement of parts, the combination thereof and mode of operation, which generally stated consist in a device capable of carrying out the objects set forth, as disclosed and defined in the appended claims.

Having thus described my invention, I claim:

1. An adjustable voltage transformer having an annular core, a winding upon the core, a rotatable'shaft concentric with the core, a hub attached to the shaft and rotatable therewith, the combination comprising an arm carried by the hub and pivotal about an axis normal to the shaft, the arm extending to a position adjacent the winding, a plurality of brush members rigidly attached to the arm and in engagement with the winding, a resilient member carried by the hub and engaging the arm and urging pivotal movement thereof in a direction to cause engagement of said brush members with the winding, a resistor carried by the arm and electrically connected to the brush members, the resistor being disposed in spaced relation from the brush members and from the winding, a connector terminal for connection to exteriorly extending conductor, and means electrically attaching the resistor to the connector terminal.

2. In an adjustable transformer, an annular core, and a toroidal winding disposed upon the core, the winding including a plurality of rigid conductor pieces joined one to the other in end to end relation forming a plurality of spaced-apart convolutions, each of the conductor pieces having straight side surfaces which extend substantially toward the center of the core, each convolution of the winding having an edge thereof provided with a flat commutator portion, a pair of arms secured one with respect to the other and movable about an axis concentric with the core, each of the arms being a conductor member, a pair of brush members, there being a brush member electrically attached to each of the arms, the brush members being electrically separated and slidingly engageable with the commutator portions of the winding, a resistor having each end thereof electrically connected to one of the arms, the resistor being disposed in spaced relation from the brush members and in spaced relation from the winding, a plurality of external lead connectors, there being at least two external lead connectors electrically attached to the winding, and means electrically connecting the resistor to an external lead connector.

3. In an adjustable voltage transformer, an annular core, a toroidal Winding having a plurality of convolutions upon the core, the winding being of material having a rectangular cross-section so that the side and edge portions ares ubstantially normal one to the other, each convolution having a portion thereof within the central opening of the annular core, each convolution having a portion outside the central opening of the core substantially parallel to the portion within the central opening of the core, the portion of each convolution within the central opening having cross-sectional dimensions different from said portion of the convolution outside the central opening, an edge portion of each convolution of the winding having a cmmutator surface, and a brush member slidably movable upon the commutator surfaces of the convolutions.

4. In an adjustable voltage transformer, an annular core, a toroidal winding having a plurality of convolutions upon the core, adjacent convolutions of the winding being in spaced-apart relation, each convolution having a portion thereof within the central opening of the annular core, each convolution having a portion outside the central opening of the core substantially parallel to the portion within the central opening of the core, the portion of each convolution within the central opening having different cross-sectional dimensions from the portion outside the central opening, each convolution having side and edge portions, each convolution having a commutator surface on an edge portion thereof, and a pair of brush members insulated one from the other and rigidly attached one to the other and slidably movable upon the commutator surface of the convolutions.

5. In an adjustable voltage transformer, an annular core, a toroidal winding having a plurality of convolutions upon the core, adjacent convolutions of the winding being in spaced-apart relation, each convolution having a portion thereof within the central opening of the annular core, each convolution having a portion outside the central opening of the core substantially parallel to the portion within the central opening of the core, the portion of each convolution within the central opening having different cross-sectoinal dimensions from the portion outside the central opening, the winding being of ma terial having straight parallel side and edge portions, the edge dimension being considerably less than the side dimensions so that the cross-section of the material is substantially oblong, an edge portion of each convolution having a commutator surface, and a pair of arm members rotatable about an axis concentric with the core, each arm member being a conductor member, and a brush attached to each arm member and slidably engageable with the commutator surfaces of the convolutions.

6. An adjustable voltage transformer comprising an annular core having a central opening, a toroidal winding disposed upon the core, the winding including a plurality of convolutions of conductor material, the conductor material having a rectangular cross-section, the conductor material having side dimensions and edge dimensions, the edge dimensions being substantially constant along the length of each convolution, each convolution having a portion thereof within the central opening of the core, each convolution also having a portion at the peripheral surface of the core and substantially parallel to the portion of the convolution which is within the central opening of the core, the side dimension of each portion of a convolution within the central opening being less than the side dimension of the portion of the convolution which is at the peripheral surface of the core, the portion of each convolution which is at the peripheral surface of the core thus serving as an additional heat radiator as well as a conductor, a portion of an edge of each convolution having a commutator surface, and a movable brush slidably engageable with the commutator surface of each convolution.

7. An adjustable voltage transformer comprising an annular core having a central opening, a toroidal winding having a plurality of convolutions disposed upon the core,

each convolution of the winding having a portion within the central opening of the core and a portion at the peripheral surface of the core and substantially parallel to the portion within the central opening, each of said portions tapering in thickness toward the center of the core, each convolution having an edge provided with a commutator surface, and a brush slidably movable from the commutator surface of one convolution to the commutator surface of an adjacent convolution.

8. An adjustable voltage transformer comprising an annular core having a central opening therein, a toroidal winding disposed upon the core, the winding having a plurality of spaced-apart convolutions, each convolution having side surfaces extending substantially radially toward the center of the core so that each convolution has tapered portions, each convolution having an edge portion provided with a commutator surface, a brush member slidably movable upon the commutator surface of the convolutions of the winding.

9. In an adjustable voltage transformer, an annular core, a toroidal Winding having a plurality of convolutions upon the core, adjacent convolutions of the Winding being in spaced-apart relation, each convolution having a portion thereof within the central opening of the annular core, each convolution having a portion outside the central opening of the core substantially parallel to the portion within the central opening of the core, the portion of each convolution within the central opening having difierent cross-sectional dimensions from the portion outside the central opening, the winding being of material having straight parallel side and edge portions, the edge dimension being considerably less than the side dimensions so that the cross-section of the material is substantially oblong, an edge portion of each convolution having a commutator surface, and a pair of arm members rotatable about an axis concentric with the core, each arm member being a conductor member, a brush attached to each arm member and slidably engageable with the commutator surfaces of the convolutions, and impedance means connected between said arm members and disposed in spaced relation from the brushes and in spaced relation from the winding.

References Cited in the file of this patent UNITED STATES PATENTS 1,801,214 Von Henke Apr. 14, 1931 2,009,013 Karplus July 23, 1935 2,092,058 Ferris Sept. 7, 1937 2,273,245 Ander Feb. 17, 1942 2,361,384 Davis Oct. 31, 1944 2,513,677 Rigert July 4, 1950 2,666,187 Ketcham Jan, 12, 1954 2,765,448 Dufiing Oct. 2, 1956 FOREIGN PATENTS 147,324 Sweden Oct. 19, 1954 

